Sram cells, memory circuits, systems, and fabrication methods thereof

ABSTRACT

A static random access memory (SRAM) cell includes a pair of cross-coupled inverters having a first node and a second node. A first transistor is coupled between the first node and a first bit line. A second transistor is coupled between the second node and a second bit line. A third transistor is coupled with the first node. The third transistor has a threshold voltage that is higher than that of a fourth transistor of the pair of cross-coupled inverters by about 10% or more. A fifth transistor is coupled between the third transistor and a third bit line

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/877,695, filed Aug. 8, 2010, which in turn claims priority of U.S.Provisional Patent Application Ser. No. 61/242,167, filed on Sep. 14,2009, which are incorporated herein by reference in their entirties.

TECHNICAL FIELD

The present disclosure relates generally to the field of semiconductorcircuits, and more particularly, to SRAM cells, SRAM circuits, systems,and fabrication methods thereof.

BACKGROUND

Semiconductor memory devices include, for example, static random accessmemory, or SRAM, and dynamic random access memory, or DRAM. DRAM memorycell has only one transistor and one capacitor, so it provides a highdegree of integration. DRAM requires constant refreshing. Also, itspower consumption and slow speed limit its use mainly for computer mainmemories. An SRAM cell, on the other hand, is bi-stable, meaning it canmaintain its state indefinitely as long as an adequate power issupplied. SRAM can operate at a higher speed and lower powerdissipation, so computer cache memories use exclusively SRAMs. Otherapplications include embedded memories and networking equipmentmemories. There are several types of SRAM cells, e.g., 6-transistor (6T)SRAM, dual-port 8-transistor (8T) SRAM, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the numbers and dimensions of the various features may bearbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a schematic drawing illustrating an exemplary static randomaccess memory (SRAM) cell.

FIG. 2 is a schematic drawing illustrating an exemplary SRAM circuitincluding two SRAM cells coupled with the same word line.

FIG. 3 is a schematic drawing illustrating a simulation result of thethreshold voltage difference by percentage (V_(th1)−V_(th2))/V_(th2)v.s. a leakage current reduction by percentage.

FIG. 4 is a schematic drawing illustrating a simulation result of thethreshold voltage difference by percentage of (V_(th1)−V_(th3))/V_(th3)v.s. a cell current increase by percentage.

FIG. 5 is a flowchart illustrating an exemplary method for forming aSRAM cell.

FIG. 6 is a schematic drawing showing a SRAM circuit including a SRAMcell coupled with a sense circuit.

DETAILED DESCRIPTION

A conventional eight-transistor (8T) SRAM cell consists of eightmetal-oxide-semiconductor (MOS) transistors. The conventional 8T SRAMcell has two identical cross-coupled inverters that form a latchcircuit, i.e., one inverter's output connected to the other inverter'sinput. The latch circuit is connected between a power and a ground. Eachinverter consists of an NMOS pull-down transistor and a PMOS pull-uptransistor. The inverters' outputs serve as two storage nodes. One ispulled to a low voltage and the other is pulled to a high voltage. Acomplementary write bit-line pair is coupled to the pair of storagenodes via a pair of write pass-gate NMOS transistors. The gates of thewrite pass-gate NMOS transistors are commonly connected to a write wordline.

The conventional 8T SRAM cell also has a read pass-gate NMOS transistorcoupled to a read bit line. A gate of the read pass-gate NMOS transistoris coupled to a read word line. An NMOS transistor has a gate coupled toone of the storage nodes. The source of the NMOS transistor is grounded.The drain of the NMOS transistor is coupled to the source of the readpass-gate NMOS transistor. Conventionally, the NMOS transistor, the NMOSpull-down transistors, the read pass-gate NMOS transistor, and the writepass-gate NMOS transistors are formed by the same channel ionimplantation and have the same threshold voltage. It is found thatwriting a datum to a conventional 8T SRAM cell may result in misreadinga datum stored within another conventional 8T SRAM cell that is disposedon the same column of the former.

Based on the foregoing, SRAM cells, SRAM circuits, systems, andfabrication methods thereof are desired.

It is understood that the following disclosure provides many differentembodiments, or examples, for implementing different features of theapplication. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a feature on, connected to, and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact. In addition, spatially relative terms, for example, “lower,”“upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,”“top,” “bottom,” etc. as well as derivatives thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of thepresent disclosure of one features relationship to another feature. Thespatially relative terms are intended to cover different orientations ofthe device including the features.

FIG. 1 is a schematic drawing illustrating an exemplary static randomaccess memory (SRAM) cell. In FIG. 1, a SRAM cell 100 can include a pairof cross-coupled inverters 101. The cross-coupled inverters 101 can havenodes N1 and N2. The SRAM cell 100 can include a transistor 130 coupledbetween the node N1 and a bit line BL2. The SRAM cell 100 can include atransistor 135 coupled between the node N2 and a bit line BL1. The SRAMcell 100 can include a transistor 140 coupled with the node N1. The SRAMcell 100 can include a transistor 150 coupled with the transistor 140and a bit line BL3. The transistor 140 can have a threshold voltageV_(th1) that is higher than a threshold voltage V_(th2) of a transistor,e.g., a transistor 125, of the pair of cross-coupled inverters 101 byabout 10% or more.

In some embodiments, the bit lines BL1 and BL2 can be a complementarybit line pair. The bit lines BL1 and BL2 can be referred to as a writebit line and a write bit line bar, respectively. The bit line BL3 can bereferred to as a read bit line. The transistors 130 and 135 can bereferred to as access transistors. In some embodiments, gates of thetransistors 130 and 135 can be coupled with a word line WL1. A gate ofthe transistor 150 can be coupled with a word line WL2. The word linesWL1 and WL2 can be referred to as a write word line and a read wordline, respectively.

Referring to FIG. 1, the pair of cross-coupled inverters 101 can includetransistors, 110, 115, 120, and 125. The transistors 110 and 115, e.g.,PMOS transistors, can be coupled with a power line that can provide avoltage state, e.g., VDD. The transistors 120 and 125, e.g., NMOStransistors, can be coupled with a power line that can provide anothervoltage state, e.g., VSS or ground. In some embodiments, the transistors110 and 115 can be referred to as pull-up transistors. The transistors120 and 125 can be referred to as pull-down transistors.

In some embodiments, the transistors 140 and 150 can be NMOStransistors. A gate of the transistor 140 can be coupled with the nodeN1. A source end of the transistor 140 can be coupled with a power linethat can provide a voltage state, e.g., VSS or ground. A drain end ofthe transistor 140 can be coupled with a source end of the transistor150. A drain end of the transistor 150 can be coupled with the bit lineBL3. It is noted that the types of the transistors 110, 115, 120, 125,130, 135, 140, and 150 described above are merely exemplary. One ofskill in the art can modify the types of the transistors to achieve adesired dual-port SRAM cell.

FIG. 2 is a schematic drawing illustrating an exemplary SRAM circuitincluding two SRAM cells coupled with the same word line. In FIG. 2, aSRAM circuit 200 can include the SRAM cell 100 described above inconjunction with FIG. 1 and another SRAM cell 201. The SRAM cell 201 canhave a cell structure similar to that of the SRAM cell 100. The SRAMcells 100 and 201 can be coupled with the word line WL1. In someembodiments, the SRAM cells 100 and 201 can be disposed on the samecolumn.

It is noted that though only two SRAM cells 100 and 201 are depicted,other cells (not shown) can be placed at the intersection of a pluralityof word lines and the bit lines. A portion of the memory circuit 200 mayhave 8, 16, 32, 64, 128 or more columns that can be arranged in wordwidths. In some embodiments, the word lines can be laid outsubstantially orthogonally to the bit lines. In some other embodiments,other arrangements of the word lines and bit lines can be provided.

Following is a description regarding an exemplary operation of the SRAMcircuit 200. In some embodiments for reading data from the SRAM cell 100and writing data into the SRAM cell 201, the bit lines BL1-BL3 can beprecharged to a voltage state, e.g., VDD. In some embodiments, the nodeN1 can store a voltage state, e.g., a low voltage state or 0, and thenode N2 can have another voltage state, e.g., a high voltage state or 1.It is noted that the precharged voltage state VDD and/or the voltagestates of the nodes N1 and N2 are merely exemplary. In some embodiments,the precharged voltage state can be ½ VDD.

In some embodiments, writing a datum to the SRAM cell 201 and readinganother datum stored within the SRAM cell 100 can be performedsimultaneously. For writing the datum to the SRAM cell 201, the voltagestate of the word line WL1 can be pulled up to a voltage state, e.g.,VDD. For reading the datum stored in the SRAM cell 100, the voltagestate of the word line WL 2 can be pulled up to a voltage state, e.g.,VDD, for turning on the transistor 150.

As noted, the SRAM cell 100 is coupled with the word line WL1. Thevoltage VDD applied to the word line WL1 can turn on the transistors 130and 135. The turned-on transistor 130 can couple the node N1 having thelow state with the bit line BL2 that is precharged to the voltage stateVDD. Since the node N1 has a low voltage state, the precharged voltagestate of the bit line BL2 can pull up the voltage state on the node N1.The pulled-up voltage state on the node N1 is coupled with the gate ofthe transistor 140.

As noted, the threshold voltage V_(th1) of the transistor 140 can behigher than the threshold voltage V_(th2) of the transistor 125 by about10% or more. The high threshold voltage V_(th1) of the transistor 140can desirably reduce the leakage current resulting from the pulled-upvoltage state on the node N1. The reduction of leakage current candesirably let a sense circuit (not shown) to sense the stored state,e.g., the low state, on the node N1, instead of the pulled-up voltagestate. The misreading of the datum stored in the SRAM cell 100 can bedesirably reduced. FIG. 3 is a schematic drawing illustrating asimulation result of the threshold voltage difference by percentage(V_(th1)−V_(th2))/V_(th2) v.s. a leakage current reduction bypercentage. In FIG. 3, the horizontal axis can represent the thresholdvoltage difference by percentage (V_(th1)−V_(th2))/V_(th2). The verticalaxis can represent a leakage current reduction by percentage. Theleakage current can include a leakage current flowing through thetransistor 140. As shown in FIG. 3, the leakage current reduction bypercentage can be substantially increased if the difference percentageof (V_(th1)−V_(th2))/V_(th2) can be about 10% or more.

Referring again to FIG. 1, in some embodiments the threshold voltageV_(th1) of the transistor 140 can be higher than a threshold voltageV_(th3) of the transistor 150 by about 10% or more. In some otherembodiments, the threshold voltage V_(th1) of the transistor 140 can behigher than the threshold voltage V_(th3) of the transistor 150 by about20% or more. In still some other embodiments, the threshold voltageV_(th3) of the transistor 150 can be substantially equal to thresholdvoltage V_(th2) of the transistor 125. In still further embodiments, thethreshold voltage V_(th3) of the transistor 150 can be lower than thethreshold voltage V_(th2) of the transistor 125 by about 10% or more.

Instead of storing a low voltage state described above, in some otherembodiments the node N1 can store a high voltage state. To sense thehigh voltage state on the node 1, the cell current is desired to reach apredetermined level. As noted, the threshold voltage V_(th1) of thetransistor 140 is higher than the threshold voltage V_(th1) of thetransistor 125 by about 10% or more. The high threshold voltage V_(th1)of the transistor 125 may reduce the cell current. To compensate the lowcell current resulting from the high threshold voltage V_(th1) of thetransistor 125, the threshold voltage V_(th3) of the transistor 150 isreduced. With the lower threshold voltage V_(th3) of the transistor 150,the cell current can be desirably achieved. The sense circuit (notshown) can desirably sense the stored high voltage on the node N1.

FIG. 4 is a schematic drawing illustrating a simulation result of thethreshold voltage difference by percentage of (V_(th1)−V_(th3))/V_(th3)v.s. a cell current increase by percentage. In FIG. 4, the horizontalaxis can represent the threshold voltage difference by percentage(V_(th1)−V_(th3))/V_(th3). The vertical axis can represent a cellcurrent increase by percentage. The cell current can include a cellcurrent flowing through the transistor 140. As shown in FIG. 4, the cellcurrent increase by percentage can be substantially enhanced if thedifference percentage of (V_(th1)−V_(th3))/V_(th3) can be about 20% ormore.

FIG. 5 is a flowchart illustrating an exemplary method for forming aSRAM cell. In FIG. 5, a method 500 can include processes 510-550. Theprocess 510 can form the pair of cross-coupled inverters 101 having thenodes N1 and N2 (shown in FIG. 1). The process 520 can form thetransistor 130 coupled between the node N1 and the bit line BL2. Theprocess 530 can form the transistor 135 coupled between the node N2 andthe bit line BL1. The process 540 can form the transistor 140 coupledwith the node N1. The transistor 140 can have the threshold voltageV_(th1) that is higher than the threshold voltage V_(th2) of thetransistor 125 of the pair of cross-coupled inverters 101 by about 10%or more. The process 550 can form the transistor 150 coupled between thetransistor 140 and the bit line BL3. In some embodiments, the processes510, 520, 530, 540, and/or 550 can have the process flows that aresimilar to each other. In some other embodiments, the processes 510,520, 530, 540, and/or 550 can be a single process that can form thecross-coupled inverters 101 and the transistors 130, 135, 140, and 150simultaneously. The processes 510, 540, and/or 550 can have differentchannel implantation processes.

In some embodiments, the substrate (not shown) over which the SRAM cell101 is formed can include an elementary semiconductor including siliconor germanium in crystal, polycrystalline, or an amorphous structure; acompound semiconductor including silicon carbide, gallium arsenic,gallium phosphide, indium phosphide, indium arsenide, and indiumantimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs,AlGaAs, GaInAs, GaInP, and GaInAsP; any other suitable material; orcombinations thereof. In one embodiment, the alloy semiconductorsubstrate may have a gradient SiGe feature in which the Si and Gecomposition change from one ratio at one location to another ratio atanother location of the gradient SiGe feature. In another embodiment,the alloy SiGe is formed over a silicon substrate. In anotherembodiment, a SiGe substrate is strained. Furthermore, the semiconductorsubstrate may be a semiconductor on insulator, such as a silicon oninsulator (SOI), or a thin film transistor (TFT). In some examples, thesemiconductor substrate may include a doped epi layer or a buried layer.In other examples, the compound semiconductor substrate may have amultilayer structure, or the substrate may include a multilayer compoundsemiconductor structure.

In some embodiments, the method 500 can include defining an oxidedefinition (OD) region (not shown). The OD region can be defined by aSTI process, a LOCOS process, or other suitable process that can form adesired isolation structure.

Areas around the OD region can include materials such as oxide, nitride,oxynitride, other dielectric material that can isolate the OD regionfrom other OD regions, and/or any combinations thereof.

In some embodiments, implantation processes can be performed to implantdopants within the OD regions to achieve desired threshold voltages ofthe transistors. As noted, the transistor 140 can have the thresholdvoltage V_(th1) that is higher than the threshold voltage V_(th2) of thetransistor 125. In some embodiments, the process 510 can includeimplanting a channel dopant in the transistor 125 such that thetransistor 125 has a first channel dopant concentration. The process 540can include implanting a channel dopant in the transistor 140 such thatthe transistor 140 can have a second channel dopant concentration. Thesecond channel dopant concentration is higher than the first channeldopant concentration. In embodiments, implanting the channel dopant inthe transistor 125 and implanting the channel dopant in the transistor140 are different implantation processes. In some other embodiments,implanting the channel dopant in the transistor 125 can also implantchannel dopant in the transistor 140. Implanting the channel dopant inthe transistor 140 can implant more dopants in the channel of thetransistor 140.

In some embodiments, the threshold voltage V_(th1) of the transistor 140is higher than a threshold voltage V_(th3) of the transistor 150 byabout 10% or more. For example, the process 550 can include implanting achannel dopant in the transistor 150 such that the transistor 150 has athird channel dopant concentration. The third channel dopantconcentration is lower than the second channel dopant concentration. Insome other embodiments, the third channel dopant concentration issubstantially equal to the first channel dopant concentration.

In some other embodiments, the threshold voltage V_(th1) of thetransistor 140 is higher than a threshold voltage V_(th3) of thetransistor 150 by about 20% or more. The third channel dopantconcentration is lower than the first channel dopant concentration.

In some embodiments, the word lines WL1 and WL2 can be formed usingprocesses such as, deposition, photolithography, wet etching, dryetching (e.g., reactive ion etch (RIE)), plasma etching, and/or othersuitable processes. The word lines WL1 and WL2 may include polysilicon,Ti, TiN, TaN, Ta, TaC, TaSiN, W, WN, MoN, MoON, RuO₂, and/or othersuitable materials. The word lines WL1 and WL2 may include one or morelayers formed by physical vapor deposition (PVD), CVD, ALD, plating,and/or other suitable processes.

In some embodiments, source/drain (S/D) regions (not labeled) of thetransistors 110, 115, 120, 125, 130, 135, 140, and 150 can be formed byimplanting dopants within the OD region. For embodiments formingN-channel memory cells, the S/D regions can have dopants such as Arsenic(As), Phosphorus (P), other group V element, or the combinationsthereof. In some other embodiments, the S/D regions can include silicidefor low resistances. The silicide may comprise materials such as nickelsilicide (NiSi), nickel-platinum silicide (NiPtSi),nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide(NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridiumsilicide (IrSi), erbium silicide (ErSi), cobalt silicide (CoSi), othersuitable materials, and/or combinations thereof. The materials utilizedto create the silicide may be deposited using PVD such as sputtering andevaporation; plating; CVD such as plasma enhanced CVD (PECVD),atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), high densityplasma CVD (HDPCVD) and atomic layer CVD (ALCVD); other suitabledeposition processes; and/or combinations thereof. After deposition, thesalicidation process may continue with a reaction between the depositedmaterial and the doped regions at an elevated temperature that isselected based on the specific material or materials. This is alsoreferred to as annealing, which may include a RTP. The reacted silicidemay require a one step RTP or multiple step RTPs.

In some embodiments, at least one dielectric structure (not shown) canbe formed over the substrate. The dielectric structure may includematerials such as oxide, nitride, oxynitride, low-k dielectric material,ultra low-k dielectric material, or any combinations thereof. Thedielectric structure may be formed by, for example, a CVD process, a HDPCVD process, a HARP, a spin-coating process, other deposition process,and/or any combinations thereof.

In some embodiments, contact plugs, via plugs, metallic regions,metallic lines, and/or the bit lines BL1-BL3 can be formed within thedielectric structure for interconnection. The contact plugs, via plugs,metallic regions, metallic lines, and/or the bit lines BL1-BL3 caninclude materials such as tungsten, aluminum, copper, titanium,tantalum, titanium nitride, tantalum nitride, nickel silicide, cobaltsilicide, other proper conductive materials, and/or combinationsthereof. The contact plugs, via plugs, metallic regions, metallic lines,and/or the bit lines BL1-BL3 can be formed by any suitable processes,such as deposition, photolithography, and etching processes, and/orcombinations thereof.

FIG. 6 is a schematic drawing showing a SRAM circuit including a SRAMcell coupled with a sense circuit. In FIG. 6, a SRAM circuit 600 caninclude a SRAM cell 601 coupled with a sense circuit 610. The SRAM cell601 can be similar to the SRAM cell 100 described above in conjunctionwith FIG. 1. The sense circuit 610 can be coupled with the SRAM cell 601through at least one of the bit lines BL1-BL3. The sense circuit 610 cansense a cell current of the SRAM cell 601 to determine the datum storedwithin the SRAM cell 601.

In some embodiments, a system can include a processor (not shown)coupled with the SRAM circuit 600. In some embodiments, the processorcan be a processing unit, central processing unit, digital signalprocessor, or other processor that is suitable for accessing data ofmemory circuit.

The processor and the SRAM circuit 601 can be formed within a systemthat can be physically and electrically coupled with a printed wiringboard or printed circuit board (PCB) to form an electronic assembly. Theelectronic assembly can be part of an electronic system such ascomputers, wireless communication devices, computer-related peripherals,entertainment devices, or the like.

In some embodiments, the system including the SRAM circuit 600 canprovides an entire system in one IC, so-called system on a chip (SOC) orsystem on integrated circuit (SOIC) devices. These SOC devices mayprovide, for example, all of the circuitry needed to implement a cellphone, personal data assistant (PDA), digital VCR, digital camcorder,digital camera, MP3 player, or the like in a single integrated circuit.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A static random access memory (SRAM) cell comprising: a pair ofcross-coupled inverters having a first node and a second node; a firsttransistor coupled between the first node and a first bit line; a secondtransistor coupled between the second node and a second bit line; athird transistor having a gate coupled with the first node; and a fourthtransistor coupled between the third transistor and a third bit line,the third transistor having a threshold voltage that is higher than thatof the fourth transistor by about 10% or more.
 2. The SRAM cell of claim1, wherein the threshold voltage of the third transistor is higher thanthe threshold voltage of the fourth transistor by about 20% or more. 3.The SRAM cell of claim 1, wherein the threshold voltage of the thirdtransistor is higher than a threshold voltage of a fifth transistor ofthe pair of cross-coupled inverters by about 10% or more.
 4. The SRAMcell of claim 3, wherein the threshold voltage of the fourth transistoris substantially equal to the threshold voltage of the fifth transistor.5. The SRAM cell of claim 3, wherein the threshold voltage of the fourthtransistor is lower than the threshold voltage of the fifth transistorby about 10% or more.
 6. The SRAM cell of claim 3, wherein a source ofthe third transistor and a source of the fifth transistor are coupled toa power node.
 7. The SRAM cell of claim 6, wherein the third transistorand the fifth transistor are NMOS transistors.
 8. A static random accessmemory (SRAM) circuit comprising: a sense amplifier; and a SRAM cellcoupled with the sense amplifier, the SRAM cell comprising: a pair ofcross-coupled inverters having a first node and a second node; a firsttransistor coupled between the first node and a first bit line; a secondtransistor coupled between the second node and a second bit line; athird transistor having a gate coupled with the first node; a fourthtransistor coupled between the third transistor and a third bit line,the third transistor having a threshold voltage that is higher than thatof the fourth transistor by about 10% or more.
 9. The SRAM circuit ofclaim 8, wherein the threshold voltage of the third transistor is higherthan the threshold voltage of the fourth transistor by about 20% ormore.
 10. The SRAM circuit of claim 8, wherein the threshold voltage ofthe third transistor is higher than a threshold voltage of a fifthtransistor of the pair of cross-coupled inverters by about 10% or more.11. The SRAM circuit of claim 10, wherein the threshold voltage of thefourth transistor is substantially equal to the threshold voltage of thefifth transistor.
 12. The SRAM circuit of claim 8, wherein the thresholdvoltage of the fourth transistor is lower than the threshold voltage ofthe fifth transistor by about 10% or more.
 13. A method of forming astatic random access memory (SRAM) cell, the SRAM cell comprising a pairof cross-coupled inverters, a first transistor coupled between a firstnode of the pair of cross-coupled inverters and a first bit line, asecond transistor coupled between a second node of the pair ofcross-coupled inverters and a second bit line, a third transistor havinga gate coupled with the first node, and a fourth transistor coupledbetween the third transistor and a third bit line, the methodcomprising: forming an isolation structure to define a plurality ofoxide definition (OD) regions; performing implantation processes to seta threshold voltage of the third transistor higher than that of thefourth transistor by about 10% or more; forming source and drain regionsin the OD regions for the cross-coupled inverters and the first, second,third, and fourth transistors; and forming an interconnection structureover the OD regions.
 14. The method of claim 13, wherein the thresholdvoltage of the third transistor is higher than the threshold voltage ofthe fourth transistor by about 20% or more.
 15. The method of claim 13,wherein the performing implantation processes comprises implanting achannel dopant for the third and fourth transistors with differentchannel dopant concentrations.
 16. The method of claim 13, wherein thethreshold voltage of the third transistor is set to be higher than athreshold voltage of a fifth transistor of the cross-coupled invertersby about 10% or more.
 17. The method of claim 16, wherein the performingimplantation processes comprises implanting a channel dopant for thethird and fifth transistors with different channel dopantconcentrations.
 18. The method of claim 16, wherein the performingimplantation processes comprises implanting a channel dopant for thefourth and fifth transistors with a same channel dopant concentration.19. The method of claim 16, wherein the implantation processes and theformation of the source and drain regions are set to form the third andfifth transistors as NMOS transistors.
 20. The method of claim 13,wherein the implantation processes and the formation of the source anddrain regions are set to form the third and fourth transistors as NMOStransistors.